Artist's concept of Eris and Dysnomia.appear in the distance. Credit: R. Hurt, IPAC
Dysnomia (formally (136199) Eris I Dysnomia) is the only known moon of the dwarf planet Eris and likely the second-largest known moon of a dwarf planet, after Pluto I Charon. It was discovered in 2005 by Mike Brown and the laser guide star adaptive optics team at the W. M. Keck Observatory, and carried the provisional designation of S/2005 (2003 UB313) 1 until officially named Dysnomia (from the Ancient Greek word Δυσνομία meaning anarchy/lawlessness) after the daughter of the Greek goddess Eris.
Dysnomia has an estimated diameter of 700±115 km (25% to 35% of Eris's diameter), and is among the dozen or so largest objects in the trans-Neptunian region.
Dysnomia has an estimated diameter of 700 ± 115 km with an albedo of 0.04+0.02 −0.01. The estimate was obtained using radiometric observation by ALMA observatory in submillimeter spectral region. The same study suggested a wide range of potential masses for the satellite, depending on its actual density, with a system mass ratio anywhere from 37:1 to 115:1 (thus masses of approximately 0.143×1021 kg with a minimal 0.8 g/cm3 density, to 0.437×1021 kg for the same density as Eris itself).
Although the shape of Dysnomia is not known, it is presumed to have a spherical shape due to its large dimensions; it is larger than the three smallest ellipsoidal moons of Saturn and Uranus (Enceladus, Miranda, and Mimas).
In its discovery images Dysnomia was ~60 times fainter (or 4.43 magnitudes) than Eris in the K band, and later observations with the Hubble Space Telescope found it to be 500 times fainter in the visible band.This indicates a very different, and quite redder, spectrum, indicating a significantly darker surface. Its diameter reveals it to be a rather large trans-Neptunian object. Of the known moons of dwarf planets, only Charon is larger than Dysnomia.
Further Data
Mike Brown, the moon's discoverer, chose the name Dysnomia for the moon. As the daughter of Eris, the mythological Dysnomia fit the established pattern of naming moons after gods associated with the primary body (hence, Jupiter's largest moons are named after lovers of Jupiter, while Saturn's are named after his fellow Titans). Also, the English translation of "Dysnomia", "lawlessness", echoes Lucy Lawless, the actress who played Xena in Xena: Warrior Princess on television. Before receiving their official names, Eris and Dysnomia had been nicknamed "Xena" and "Gabrielle", though Brown states that the connection was accidental.
A primary reason for the name was its similarity to the name of Brown's wife, Diane, following a pattern established with Pluto. Pluto owes its name in part to its first two letters, which form the initials of Percival Lowell, the founder of the observatory where its discoverer, Clyde Tombaugh, was working, and the person who inspired the search for "Planet X". James Christy, who discovered Charon, did something similar by adding the Greek ending -on to Char, the nickname of his wife Charlene. (Christy wasn't aware that the resulting 'Charon' was a figure in Greek mythology.) "Dysnomia", similarly, has the same first letter as Brown's wife, Diane, and Brown uses the nickname "Dy" /ˈdaɪ/ for the moon, which he pronounces the same as his wife's nickname, Di. Because of this, Brown pronounces the full name /daɪsˈnoʊmiə/, with a long "y" sound.
